The present invention provides a catalyst and process for polymerizing olefins having three or more carbon atoms to produce a polymer with a syndiotactic stereochemical configuration. The catalyst and process are particularly useful in polymerizing propylene to form a highly crystalline, novel microstructure of syndiotactic polypropylene.
As known in the art, syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the macromolecular main chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. The Natta group obtained syndiotactic polypropylene by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or haloenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a phosphorus-containing Lewis base as producing syndiotactic polypropylene.
As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is ...mmmm... with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene is shown in zig-zag representation as follows: ##STR2## Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: ##STR3## In NMR nomenclature, this pentad is described as ... rrrr... in which each "r" represents a "racemic" dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
While it is possible for a catalyst to produce all three types of polymer, it is desirable for a catalyst to produce predominantly isotactic or syndiotactic polymer with very little atactic polymer. Catalysts that produce isotactic polyolefins are disclosed in copending U.S. Patent Application Ser. Nos. 034,472 filed Apr. 3, 1987 and now abandoned; 096,075 filed Sept. 11, 1987 and now Pat. No. 4,794,096; and 095,755 filed on Sept. 11, 1987 and now abandoned. These applications disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of a highly isotactic polypropylene. The present invention, however, provides a different class of metallocene catalysts that are useful in the polymerization of syndiotactic polyolefins, and more particularly syndiotactic polypropylene.
In addition to a newly discovered catalyst, the present invention also provides syndiotactic polypropylene with a new microstructure. It was discovered that the catalyst structure not only affected the formation of a syndiotactic polymer as opposed to an isotactic polymer, but it also appears to affect the type and number of deviations in the chain from the principally repeating units in the polymer. Previously, the catalysts used to produce syndiotactic polypropylene were believed to exercise chain-end control over the polymerization mechanism. These previously known catalysts, such as the ones disclosed by Natta et al in the references cited above, produce predominately syndiotactic polymers having the structure ##STR4## or in NMR nomenclature ...rrrrrmrrrrr.... The NMR analysis for this structure of syndiotactic polypropylene is shown in Zambelli, et al., Macromolecules, Vol. 13, pp 267-270 (1980). Zambelli's analysis shows the predominance of the single meso dyad over any other deviation in the chain. It was discovered, however, that the catalysts of the present invention produce a polymer with a different microstructure than that previously known and disclosed, and in addition one having a high percentage of racemic dyads in the structure.